Theory:

Pure substances are those that are completely made up of one form of particle. Physical processes cannot distinguish pure substances from other forms of matter.
Example:
Salt, sugar, oxygen, copper, iron etc.,
Based on the kind of atom, we classify pure substances as
  • Elements and
  • Compounds
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Compound is a form of matter created by combining two or more elements in a specific mass ratio. To decompose a compound into its constituent components chemical methods are used.
Water\(H_2O\), Oxygen \(O_2\), Nitrogen dioxide \(NO_2\), Salt \(NaCl\), and so on.
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Elements are the fundamental substances that cannot be separated into simpler substances by any chemical methods. All the elements in the periodic table comes under this category such as helium, hydrogen, oxygen, iron, etc.
In \(1661\), Robert Boyle became the first scientist to use the term element.
 
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Robert Boyle
 
A French chemist named Antoine Laurent Lavoisier \(1743\) - \(1794\) was the first to define an experimentally useful description of an element.
 
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Antoine Laurent Lavoisier
An element, according to him, is a fundamental type of matter that cannot be broken down into simpler substances by chemical reactions.
Based on the physical and chemical properties, elements are classified as
  • Metals
  • Non-metals
  • Metalloids
Let us now identify some of the elements and compound of a pure substance:
  
Pure Substance
Element /Compound
Reason why it is compound or element
Water \(H_2O\)Compound
Can be separated as
Water = Hydrogen + Oxygen
Lead \(Pb\)Element
Cannot be separated
Phosphorous \(P_4\)Element
Cannot be separated
Sodium Chloride \(NaCl\)Compound
Can be separated as
Sodium Chloride = Sodium + Chloride
Sulphur \(S_8\)Element
Cannot be separated